The present invention relates to an apparatus for producing an optical disk which is suitable for use as a medium for storage, recording and reproduction of information such as voice information and image information.
In general, an optical disk has a transparent replica substrate made of a transparent material such as a sheet of glass or plastics having a information pattern presented by consecutive protrusions and recesses in the form of grooves or pits formed therein. A recording metal layer and a protection layer are successively formed on the replica substrate and two such replica substrates are bonded together through an adhesive agent layer.
Hitherto, optical disks of the kind described have been produced by using a press machine which bonds two replica substrates together through a thermoplastic hot-melt adhesive agent in the atmosphere, as disclosed in Japanese Patent Examined Publication No. 63-67258, in order to simplify the production process and to reduce the cost of production. Alternatively, an apparatus which employs a combination of a vacuum chamber and a press machine has been used for bonding the replica substrates, as disclosed in Japanese Patent Unexamined Publication No. 61-50231 and Japanese Patent Examined Publication No. 3-70296.
In recent years, there is an increasing demand for higher speed of processing of information, which in turn requires high-speed driving of optical disks. This has given rise to a demand for optical disks which are not easily deformed even when driven at high speeds. The conventional production method, which relies upon bonding in the atmosphere by a press machine, tends to allow generation of voids and cavities between the adhesive layers, with the result that the closeness of bonding between two replica substrates is impaired. In addition, optical disks produced by such a conventional process tends to suffer from corrosion of the recording layer, deterioration in the adhesive agent and other problems due to deformation and invasion of moisture into the voids and cavities, which occur during shelving or driving of the disks. The alternative method which employs the combination of a vacuum chamber and a press machine also is disadvantageous in that it requires a large-size vacuum chamber for the purpose of accommodating the press machine. It is difficult to evacuate such a large vacuum chamber in a short time. In addition, the press machine also has to be large in size, in order to deform the vacuum chamber. Consequently, the whole production apparatus is required to have an impractically large size.
Thus, the known production methods require improvements in terms of the quality of the product disks and the time required for the production, and so forth.